Small speed up to StreamWriter.__init__

[bdraco]

Use classvar defaults to avoid __init__ overhead. AbstractStreamWriter already does this so we can drop a few duplicates as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.73%. Comparing base (5f5729d) to head (e20a6c1).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10038      +/-   ##
==========================================
- Coverage   98.73%   98.73%   -0.01%     
==========================================
  Files         121      121              
  Lines       36769    36766       -3     
  Branches     4392     4392              
==========================================
- Hits        36304    36301       -3     
  Misses        314      314              
  Partials      151      151              

Flag	Coverage Δ
CI-GHA	98.62% <100.00%> (-0.01%)	⬇️
OS-Linux	98.31% <100.00%> (-0.01%)	⬇️
OS-Windows	96.14% <100.00%> (-0.01%)	⬇️
OS-macOS	97.41% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.26% <100.00%> (+<0.01%)	⬆️
Py-3.10.15	97.85% <100.00%> (-0.01%)	⬇️
Py-3.11.10	97.84% <100.00%> (-0.01%)	⬇️
Py-3.11.9	97.31% <100.00%> (+<0.01%)	⬆️
Py-3.12.7	98.37% <100.00%> (-0.01%)	⬇️
Py-3.13.0	98.35% <100.00%> (-0.01%)	⬇️
Py-3.9.13	97.18% <100.00%> (+<0.01%)	⬆️
Py-3.9.20	97.76% <100.00%> (-0.01%)	⬇️
Py-pypy7.3.16	97.33% <100.00%> (-0.01%)	⬇️
VM-macos	97.41% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.31% <100.00%> (-0.01%)	⬇️
VM-windows	96.14% <100.00%> (-0.01%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #10038 will not alter performance

_{Comparing speed_up_streamwriter_init (e20a6c1) with master (5f5729d)}

Summary

✅ 44 untouched benchmarks

[patchback]

Backport to 3.11: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.11/6f4e9615d74c7d4935c11a2c9497cf6aae7149f3/pr-10038

Backported as #10039

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.12: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.12/6f4e9615d74c7d4935c11a2c9497cf6aae7149f3/pr-10038

Backported as #10040

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.